Electrophotographic processing is commonly performed by a precedure in which a latent electrostatic image is formed by various electrical means on a photoreceptor made of selenium or some other suitable photoconductive materials and toner particles are deposited on the latent image by a suitable method of development, such as a magnetic brush method, to produce a visible image.
In the development step, carrier particles are used in order to impart an appropriate amount of positive or negative electrical charge to the toner. Various types of carriers have been developed and used commercially.
While carriers are required to possess various characteristics, particularly important requirements include appropriateness of the polarity of charges generated by electrification, high impact and wear resistance, efficiency in development, and long developer life. In these respects, conventional carriers are still inadequate, and a product having satisfactory characteristics has not yet been achieved. For example, iron oxide powders and other electrially conductive carriers are capable of producing solid developed images of high quality but are not equally effective in reproducing fine lines of good quality. Furthermore, they require a special charge control agent to be incorporated in the toner in order to extend the life of the developer. Developers using coated carriers (i.e., with an insulating coating) have an extended life and the capability of reproducing fine lines of good quality, but they are ineffective in reproduction of solid developed images of high quality. With a view to solving these problems, microtoning carriers, i.e., carriers consisting of small-diameter particles in which fine magnetic particles are dispersed in a binder resin have been proposed and commercialized. However, they have not completely solved the problem of short developer life, because small-diameter carrier particles have a great tendency to adhere to the surface of the photoreceptor; their chargeability varies under hot and dry conditions on account of magnetic particles that have separated from the carrier surface; and they are not highly responsive to surface treatments. While adhesion can be prevented by employing larger particles in the carrier, as a result its chargeability is reduced, causing problems such as fogging and fouling of the interior of a copying or recording machine.